In an image forming apparatus of an electro-photographic system such as a copying machine, a printer or the like, a pressurized developing method is known as a developing method in which toners are supplied to a photosensitive drum or the like carrying a latent image thereon and attached to the latent image of the photosensitive drum to visualize the latent image. In the pressurized developing method, for example, development is conducted by charging the photosensitive drum at a constant electric potential, then forming an electrostatic latent image on the photosensitive drum by an exposure machine, contacting a developing roller carrying toners with the photosensitive drum carrying the electrostatic latent image to attach the toners to the electrostatic latent image on the photosensitive drum.
Moreover, a corona discharge system has hitherto been adopted for charging the photosensitive drum. However, it is necessary to apply a high voltage of 6 to 10 kV in the corona discharge system, which is not preferable in view of ensuring the safety of the apparatus and is not also preferable environmentally since harmful substance such as ozone or the like occurs during corona discharge. On the contrary, there is proposed a contact charging system in which a charging roller is contacted with a photosensitive drum and a voltage is applied between the photosensitive drum and the charging roller to charge the photosensitive drum.
Since the developing roller in the pressurized developing method and the charging roller in the contact charging system must be surely held in contact with the photosensitive drum while rotating, they have a structure in which a semi-conductive elastic layer composed of a semi-conductive elastomer obtained by dispersing carbon black or metal powder into an elastomer such as silicone rubber, acrylonitrile-butadiene rubber (NBR), ethylene-propylene-diene rubber (EPDM), epichlorohydrin rubber (ECO) or the like or a foam formed by foaming such an elastomer is formed on the outer periphery of the shaft composed of a conductive material such as a metal or the like. Also, there is some case that a resin coating layer is further formed on the surface of the elastic layer for the purpose of controlling charging and adhesive properties to toner, preventing the elastic layer from contaminating the photosensitive drum and so on.
Furthermore, the conductive roller in which the semi-conductive elastic layer is formed on the outer periphery of the shaft and the resin coating layer is further formed on the surface of the elastic layer is used for a toner feed roller for feeding toners to the developing roller, a transfer roller for transferring toners attached to the latent image on the photosensitive drum to a recording medium, a cleaning roller for removing the remaining toners on the photosensitive drum after transferring and the like as well as the developing roller and the charging roller.
Heretofore, the resin coating layer in the conductive roller is formed by dipping a main body of the conductive roller comprising the shaft and the elastic layer into a solvent-based or a water-based coating liquid or spraying such a coating liquid onto the main body of the roller, and then drying and curing by heat or hot air. In this case, however, it is necessary to dry for a long time, so that its commercial production requires a long drying line. Moreover, although the resin coating layer requires subtle conductivity and surface condition depending on its application, since differences of a temperature distribution, an airflow amount and the like in the drying line have a large effect on the properties of the resin coating layer, there is a problem in quality. On the contrary, there is proposed a technique in which a resin coating layer comprising an ultraviolet-curing type resin is formed on the surface of the elastic layer of the conductive roller (see JP-A-2002-310136).